1) Field of the Invention
The present invention relates to a lead frame manufacturing method and, more particularly, a method of manufacturing a lead frame employed in a leadless package such as QFN, SON, etc.
2) Description of the Related Art
In recent years, development of the LSI technology as the key technology for implementing the multimedia devices is advanced steadily toward the higher speed and the larger capacity of the data transmission. The higher density of the packaging technology serving as the interface between the LSI and the electronic device is also advanced with the advance of such development.
As the package corresponding to the high-density packaging, various packages are developed up to now. As the package for the high-density packaging using the lead frame, the leadless package such as QFN (Quad Flat Non-leaded Package), SON (Small Outline Non-leaded Package), etc, in which the leads are not extended outward, for example, is well known.
FIG. 1A is a sectional view showing an example of the semiconductor device in which an IC chip is mounted on the leadless package, and FIG. 1B is an enlarged sectional view taken along a IIxe2x80x94II line in FIG. 1A. FIG. 2A is a sectional view showing an example of the lead portions of the lead frame employed in the leadless package, and FIG. 2B is a sectional view taken along a IIIxe2x80x94III line in FIG. 2A.
As shown in FIG. 1A, in a semiconductor device 110 having a QFN structure, an IC chip 104 is mounted on a die pad 102 and also a connection electrode of this IC chip 104 and a lead portion 100 are connected electrically to each other via a wire 108. Then, this IC chip 104 is sealed with molding resin 106 that covers up to a top end portion of the lead portion 100.
In this manner, in the semiconductor device 110 having the QFN structure, an upper surface and side surfaces of the lead portion 100 are covered with the lead portion 100. Therefore, if a sectional shape of this lead portion 100 is a vertical shape, it is possible that the lead portion 100 buried in the molding resin 106 is come off from the molding resin 106.
Therefore, as shown in FIG. 2A and FIG. 2B, a sectional structure of the lead portion 100 of the lead frame is shaped into such a stepped shape that a width of a surface (surface that is connected to the connection electrode of the IC chip) is thicker than a width of a back surface (surface that is connected to the wiring substrate, or the like).
As a result, as shown in FIG. 1B, the molding resin 106 is buried to cut into the side surface of the lead portion 100. Thus, coming-off of the lead portion 100 from the molding resin 106 can be prevented by the so-called anchor effect.
In the prior art, as the method of forming the lead portion 100 having the above sectional structure, mainly wet etching is employed. If wet etching is employed, a substantially same pattern of a resist film is formed on both surfaces of a metal plate, and then the metal plate is etched from both sides by the drug using the pattern of the resist film as a mask. At this time, if etchings of the front and back surfaces of the metal plate are not executed under the same conditions and, for example, such a condition is set that etching of the back surface is executed excessively, the lead portion in which the width of the surface is thicker than the width of the back surface can be formed.
However, the method of working the metal plate by wet etching has drawbacks such that a cost is expensive and a working speed is slow. Hence, stamping using the die, which makes possible a low cost and a quick working speed, is gradually employed.
FIGS. 3A to 3C are views showing a method of forming a stepwise shape, etc. in the lead portion by the stamping (prior art). FIG. 3A is a sectional view taken along a IV-IV line in FIG. 3C.
In the method of forming the stepwise shape, etc. in the lead portion by the stamping in the prior art, as shown in FIGS. 3A to 3C, first the lead portion 100, which has crushed margin portions 100a in predetermined portions on both side surfaces, is formed by punching predetermined portions of the metal plate by the stamping. Then, as shown in FIG. 3B, the die having a punch 112 and a supporting member 114 is prepared and then the crushed margin portions 100a are pushed by using the punch 112 of the die to reduce a thickness. Thus, the stepped shape is formed from both side surface portions to the center portion of the lead portion 100.
In this manner, in the prior art, the crushed margin portion 100a is provided in the predetermined portions on both side surfaces of the lead portion 100, and then the crushed margin portions 100a are pushed/crushed by the punch 112 of the die to shape the lead portions into the stepped shape.
FIGS. 4A to 4D are views showing problems in the method of forming the stepwise shape, etc. in the lead portion by the stamping in the prior art. As shown in FIGS. 4A and 4C, if a crushed portion 100b is formed while setting a crushed depth of the crushed margin portions 100a, which is crushed by the punch 112, shallow (e.g., about ⅓ of a metal plate thickness), spread of the metal is small on the surface S (surface that is connected to the connection electrode of the IC chip) side of the lead portion 100 and thus a sufficient bonding area cannot be assured. Also, as shown in an A portion of FIG. 4A, if the crushed depth is shallow, only an upper portion of the crushed portion 100b is extended in the lateral direction to form a shaving shape and thus the shape to attain a desired anchor effect cannot be obtained.
In contrast, as shown in FIGS. 4B and 4D, if the crushed portion 100c is formed while setting the crushed depth of the crushed margin portions 100a, which is crushed by the punch 112, deep (e.g., about xc2xd or more of the metal plate thickness), the sufficient bonding area can be assured on the surface S of the lead portion 100, nevertheless there is such a possibility that the crushed portion 100c that is pushed/spread by the punch 112 is spread excessively to contact mutually.
As the countermeasure against the above, there is such a method that the crushed portion 100c is formed by pushing the crushed margin portions 100a up to a depth at which a plurality of lead portions 100 come into contact with each other or come close to each other and then top end portions of a plurality of neighboring crushed portion 100c (portions surrounded by a chain line in FIG. 4D) are punched by the punch of the die such that a plurality of lead portions 100 can assure a desired interval therebetween.
However, since the portions that are punched by the punch are not united together, a punched metal residuum is separated into two pieces freely after the punching. As a result, the punched metal residuum does not stick to the side surface of the supporting member of the die but to the punch, and thus the so-called metal residuum lifting is generated.
In addition, a width of the crushed margin portion 100a of the lead portion 100 will be mentioned. If the width of the crushed margin portion 100a is narrow, the top end portion of the crushed portion is readily shaped into the above shaving shape. If the width of the crushed margin portion 100a is wide, an interval between neighboring lead portions 100 cannot be sufficiently assured since a pitch of the lead portions is decided previously. That is, it is extremely difficult to set the width of the crushed margin portion 100a that makes it possible to get the lead portion 100 with a desired sectional shape.
It is an object of the present invention to provide a lead frame manufacturing method capable of manufacturing a lead frame, which is employed in a leadless package and has lead portions each having a desired sectional shape, by the stamping not to cause any problem.
The present invention provides a lead frame manufacturing method of manufacturing a lead frame having lead portions, which extend inward from a frame portion like teeth of a comb and each of which has a base portion connected to the frame portion, a center portion connected to the base portion, and side surface portions connected to both side surfaces of the center portion to have a thickness that is smaller than other portions, by a stamping, which comprises the steps of forming a plurality of opening portions, which extend in an extending direction of the lead portions, by punching predetermined portions of a metal plate by virtue of the stamping; forming crushed portions, which extend from crushed margin portions to insides of the opening portions, by pushing the crushed margin portions, which are defined in predetermined portions of the metal plate in vicinity of the opening portions, by virtue of the stamping to reduce a thickness; defining a width between the side surface portion and the center portion to assure an interval between the lead portions and also defining a width of a top end portion connected to the center portion and a width of the base portion, by punching center portions of the crushed portions except predetermined both-side portions and predetermined portions of the metal plate in vicinity of peripheral portions, in which the crushed portions of the opening portions are not present, by virtue of the stamping; and defining the top end portions by punching a predetermined portion of the top end portion by virtue of the stamping.
As described above, in the prior art, the lead portions having the crushed margin portions in both center side-surface portions are formed previously, and then the stepwise shape, or the like is formed by crushing the crushed margin portions. Therefore, there is such a problem that the desired shape cannot be obtained if the crushed depth is set slightly shallowly whereas the desired interval between a plurality of lead portions cannot be assured if the crushed depth is set slightly deep.
In the present invention, as exemplified in FIGS. 5A to 5F and FIGS. 6A to 6H, first a plurality of opening portions 20a that extend long and narrowly in the extending direction of the lead portions are formed by punching the predetermined portions of the metal plate 20 by virtue of the stamping. Then, for example, the crushed margin portions 20x that are defined in vicinity of both edge portions of the center portions of the opening portions 20a of the metal plate 20 are pushed by the stamping to spread from both edge portions of the opening portions 20a to the inside, and thus the crushed portions 21 are formed.
Then, predetermined center portions of the crushed portions 21 and predetermined portions of the metal plate 20 in vicinity of peripheral portions of the opening portions 20a, in which the crushed portions 21 are not present, are punched by the stamping. Thus, a width W2 of the top end portion 30a of the lead portion, a width W3 of the center portion 30b having the side surface portions 30c, and a width W4 of the base portion 30d can be defined, and simultaneously an interval W6 between a plurality of neighboring lead portions can be assured. Then, the predetermined portions of the top end portions 30a are punched by the stamping and cut off from the metal plate 20, whereby the lead frame 40 is manufactured.
In this manner, in the present invention, the predetermined opening portions 20a are formed in the metal plate 20, and then the crushed portions 21 are formed by crushing the crushed margin portions 20x, which are defined in vicinity of both edge portions of the opening portions 20a, to spread to the insides of the opening portions 20a. Then, the predetermined portions of the metal plate are punched by the stamping to define widths of respective portions of the lead portions 30 and to assure the interval between a plurality of lead portions.
Accordingly, the lead portions 30 each consisting of the base portion 30d, the center portion 30b having the side surface portions 30c on both side surfaces, and the top end portion 30a are formed, and also a part of the crushed portions 21 serves as the side surface portions 30c of the lead portions 30. Therefore, the stepwise shape, for example, is formed to extend from the side surface portion 30c to the center portion 30b. 
If doing this, there is no need of consideration to assure the intervals between a plurality of lead portions when the crushed margin portions 20x of the metal plate 20 are crushed. Thus, the areas of the crushed portions 21 can be increased by setting the deepish depth of the crushed margin portions 20x. Therefore, the sufficient bonding areas can be assured in the lead portions 30, and also the stepwise shape, or the like having the sufficient anchor effect can be easily formed to extend from the center portion 30b to the side surface portion 30c of the lead portion 30. Also, after the crushed portions 21 are formed, the desired interval between a plurality of lead portions are assured by punching the predetermined portions of the metal plate. Therefore, there is no possibility that a plurality of lead portions are brought into contact with each other.
In the above lead frame manufacturing method, it is preferable that, in the step of defining a width between the side surface portion and the center portion to assure an interval between the lead portions and also defining a width of a top end portion and a width of the base portion, the stamping should be executed by using a die having punches and a supporting member, and a punched metal residuum that is punched by the punch should have such a shape that a peripheral side surface portion of the punched metal residuum comes into contact with a side surface portion of the supporting member of the die over an entire surface.
In this fashion, in the steps of defining the widths of respective portions of the lead portions and the interval between a plurality of lead portions after the opening portions 20a and the crushed portions 21 are formed, the metal plate is punched to have such a shape (a thick frame area 21a in FIG. 6A) that the punched metal residuum is brought into contact with the side surface of the supporting member 28b of the die 22b over the entire peripheral side surface portion. Thus, such a possibility can be eliminated that the metal residuum lifting is generated. As a result, generation of the defective indentations of the lead frames can be prevented and thus the lead frame with high reliability can be manufactured.
In this case, the above drawing numbers and the symbols are quoted to make the understanding of the present invention easy, and they should not be interpreted to limit the present invention.